Live or operational computer systems generally have latent defects—also called bugs. Defects may result in unexpected and/or undesirable computer performance and can cause program and/or system crashes.
In terms of the impact to end system users and the effort needed to identify, isolate, fix, and/or re-release in corrected form, software having defects, the more complex and costly defects are often those involving the interaction of multiple components or, in the case of large systems, the interactions of multiple subsystems.